    [Patent document 1] WO-2006-132003 A1
There is known a GPS positioning amendment method which amends a measuring position of a mobile station by determining a satellite signal undergoing an influence of the multipass with an easy and accurate method (see Patent document 1). In this method, a reference station transmits data for position amendment and a reception strength of a satellite signal to a mobile station such as a vehicle; the mobile station compares the reception strength in the reference station with the reception strength in the mobile station with respect to a satellite signal of each satellite. When the difference in such comparison is greater than a predetermined threshold value, the satellite signal of the satellite is removed from the satellite signals for positioning.
However, the technology in Patent document 1 assumes that a GPS receiver is installed outdoors while naturally not considering a signal attenuation due to a vehicle body. In contrast, when the GPS receiver is installed inside of the vehicle or vehicle body, a signal may decline because of a roof or a pillar of the vehicle; thus, the use of the threshold value of the signal strength (S/N) in the outdoors results in not determining the shielded state of the positioning satellite with sufficient accuracy, posing a problem.